The Effect of Four Surface Treatment Methods on the Shear Bond Strength of Metallic Brackets to the Fluorosed Enamel.

STATEMENT OF THE PROBLEM
Some studies have reported the bond strength to be significantly lower in fluorotic enamels than the non-fluorosed.


PURPOSE
The purpose of this study was to evaluate the shear bond streongth of metallic brackets to non-fluorosed and fluorosed teeth after different enamel conditioning.


MATERIALS AND METHOD
A total of 176 freshly extracted human premolars (88 non-fluorosed and 88 fluorosed teeth) were used in this study for bonding the metallic brackets. Teeth with moderate fluorosis were used according to Thylstrup and Fejereskov index (TFI). Eighty non-fluorosed and 80 fluorosed teeth (TFI=4-6) were randomly divided into 8 equal groups of 20 teeth each. The remaining 16 teeth were used for scanning electron microscopy observation. The enamel surface was conditioned by 4 methods: acid etching  for 30 sec, acid etching for 120 sec, air abrasion followed by acid etching, and Er: YAG laser etching followed by acid etching. The morphology of etching patterns in different groups was studied under scanning electron microscope.


RESULTS
The shear bond strength of fluorosed teeth to the brackets was significantly lower than non-fluorosed ones (p= 0.003). The shear bond strength of laser-acid groups in both non-fluorosed and fluorosed teeth was significantly lower than other groups (p< 0.001). Weibull analysis indicated that the chance of failure under the applied force was different between fluorosed and non-fluorosed group. The scanning electron microscope observations revealed that the fluorosed teeth treated with phosphoric acid had fewer irregularities compared to non-fluorosed teeth. The most irregularities were detected in the teeth conditioned with phosphoric acid for 120 seconds.


CONCLUSION
Fluorotic enamel adversely affects the bond strength of orthodontic brackets. None of the conditioning methods tested in this study could significantly improve shear bond strength of metallic brackets. Er: YAG laser conditioning followed by acid further reduced the bond strength in non-fluorosed and fluorosed teeth.


Introduction
Fluoridation is considered among the most effective tools in prevention of dental caries. [1] However, excessive levels of fluoride in water supplies of different geographic areas have been responsible for clinically unde-sirable fluorosis. [2] Fluorosed enamel consists of an outer hypermineralized layer which is acid-resistant and an underlying hypomineralized porous layer. [3][4] Currently, direct bonding of brackets and attachments is one of the most commonly used techniques in fixed ortho-dontic treatment. [5] Meanwhile, obtaining a strong and reliable adhesive bond between the tooth enamel and orthodontic brackets is of great importance in orthodontic practice. [6] While the routine method for conditioning the enamel surface is using phosphoric acid, [7] the hypermineralized enamel surface has been proved difficult to etch. [8] Some studies has found the bond strength to be significantly lower in fluorotic than the nonfluorosed enamels, [1,[9][10] while other studies declared no difference between the two. [5][6] Several methods of enamel surface treatment have been recommended by different studies for reinforcement of the bond strength to the fluorosed enamel. [8,[11][12] One of these methods is grinding the enamel which results in enhanced surface roughness of the fluorosed tooth [13] and might reinforce the bond strength to enamel. [14] Similarly, extended enamel conditioning with phosphoric acid can remove the acid resistant hypermineralized surface layer. [15][16] Air abrasion is another method that has been applied in order to strengthen the bond strength to fluorosed teeth. [17] This method, in which the enamel surface is roughened, can be used during orthodontic treatments. [18][19] Some investigations [20][21] also showed that the lasers can be effective in enamel conditioning. Different types of laser such as Er:YAG (erbium-doped yttrium aluminum garnet), Nd:YAG (neodymium-doped yttrium aluminum garnet) and ErCr: YSGG have been used for enamel conditioning in orthodontics. [22] Er:YAG laser has been approved as an effective tool for hard tissue ablation. [23] Attrill et al. [20] supported the use of Er:YAG laser as an alternative to conventional acid etching. Although, Lee et al. [23] and Ferreira et al. [24] reported that the mean bond strength have not increased after laser etching followed by acid etching.
To the best of our knowledge, no studies have evaluated the effect of laser etching on the bond strength of fluorosed teeth. Concerning the previous controversial results regarding other enamel conditioning methods and high prevalence of dental fluorosis in some regions, [4] it seems necessary to scrutinize the best method of enamel surface treatment for bracket bonding in fluorosed teeth. and the other half were moderately fluorosed (TFI=4-6).
[25] The fluorosed teeth were collected from endemic areas of fluorosis in southern parts of Iran.

Measurement of the shear bond strength
Before shear testing, all samples were kept in distilled water for 48 hours at room temperature. In order to determine the SBS, a mechanical testing machine (Instron Corp; Canton, Massachusetts, USA) applied an occlusocervical force to the upper surface of the bracket between the bracket base and the upper wing. SBS was measured at a crosshead speed of 0.5 mm/ minute. The maximum force needed for bracket debonding was recorded in Newton (N) and the SBS was calculated through dividing the force value by the bracket base area (1 MPa = 1 N/mm 2 ).

Adhesive remnant index (ARI)
After the experiment, an operator analyzed the brackets and teeth under a light stereomicroscope (Olympus SZ 6045 TR Zoomstere; Olympus Optical Co., Osaka, Japan) at 10X to determine the adhesive remnant index (ARI) according to Artun and Bergland. [26] The scoring criteria of the index were as (0) when no adhesive was remaining on the tooth surface, (1) when less than half the adhesive was remaining, (2) when more than half the adhesive was remaining, and (3) when the entire adhesive was remaining.

Scanning electron microscopy (SEM) observation
The roots were cut and then facial surfaces were treated similar to the enamel preparation that was performed for each of the 8 groups; 2 teeth for each enamel conditioning method. The prepared samples were washed with acetone solution for 10 minutes. Then, the enamel surfaces were sputter-coated with gold (SC-701AT; quick Auto coater, Sanyu Electron Inc, Tokyo, Japan) and observed under a SEM (JSM 5600 LV; JEOL, Tokyo, Japan) at 20KW and 1500X magnification.

Statistical analysis
The mean, standard deviation, minimum and maximum values were measured for all test groups. Two-way ANOVA was used to compare SBS among the groups.
Weibull analysis was done to calculate the Weibull modulus, characteristic strength, and the required stress for 5% and 10% probabilities of bond failure. The Chisquare test was used to determine significant differences in the ARI scores among the groups. The statistical tests were performed with SPSS software, version 17.0 (SPSS Inc; Chicago, Illinois, USA), with the significance level set at 0.05.

Shear bond strength
According to the results of two-way ANOVA test as displayed in Table 1, the SBS in the fluorosed teeth was significantly lower than non-fluorosed teeth (p< 0.05).   (Figures 1a and 1e).
Most irregularities were observed in the samples conditioned with phosphoric acid for 120 seconds (Figures 1b   and 1f). Laser-acid etched teeth showed an indistinct  etch pattern with surface cracking (Figures 1d and 1h).

Adhesive remnant index (ARI)
Modes of failure following the SBS test are summarized in Table 3. Chi-square test revealed a significant difference among the groups in terms of bond failure (p< 0.001). There were 90% and 100% frequency of ARI scores 0 and 1 in laser-acid etched groups in nonfluorosed and fluorosed teeth, respectively. But in other groups, the frequency of ARI scores was scattered.

Discussion
In this study the teeth were categorized according to the TFI. and Isci et al. [6] Moreover, in contrast to the present study, Ng'ang'a et al. [5] used tensile force for measurement of the bond strength.
In our study, the highest mean SBS on debond- Ateyah and Akpata [8] used both anterior and posterior teeth (incisor, premolar, and molar) in their study and found that the fluoride content varied among different types of the teeth. [3] In addition, they grinded to flatten the hypermineralized surface layer before acid etching. The differences between the findings of Ateyah and Akpata [8] and our study may be related to the different techniques employed as mentioned above.
The findings of the current study showed that air abrasion followed by acid etching could increase SBS value compared with acid etching alone for both fluorosed and non-fluorosed teeth; although this difference was not statistically significant. Silva-Benitez et al. [15] detected that the use of air abrasion followed by acid in severe fluorosis provided adequate SBS for fixed orthodontic appliance, but the use of this treatment could not improve bond strength in case of moderate fluorosis. Suma et al. [19] stated that combining air abrasion with acid etching created greater SBS than acid etching alone in moderate to severe dental fluorosis regardless of the adhesion system used. They used air abrasion at air pressure of 80 PSI, and the teeth were etched for 60 seconds after air abrasion. The duration of acid etching and air pressure employed in this study was higher than our study and these may be the factors responsible for different result.
Findings of the current study demonstrated that bond strengths were significantly weaker when the tooth surfaces were prepared with the Er:YAG laser followed by acid etch compared with other enamel surface treatments; it was in line with the findings of Lee et al. [23] This might be due to the reduced surface area and pore volume in the enamel surface after laser ablation. [29] The lower bond strength observed in the laser followed by acid groups compared with acid alone in our study might be attributed to the acid resistant layer created in enamel surface after laser application as indicated in some investigations. [30][31]  [20] Therefore, the energy employed in different studies.
The Weibull analysis is a suitable means for predicting the likelihood of failure of bracket bonding. [35] The results of Weibull analysis in our study showed that the Weibull modulus of non-fluorosed teeth was generally higher than that of fluorosed teeth ( According to these findings it can be affirmed that laser followed by acid etching cannot be a good choice for enamel preparation. Our SEM observations revealed that the nonfluorosed teeth treated for 30 seconds produced a pat-tern where the prism cores were eliminated and the adjacent zone did not conform to the prism structure ( Figure 1a).However, this etching time for fluorosed teeth, that are more resistant to acid, created less roughness in the enamel surface (Figure 1e). The lower microporosities in the fluorosed enamel leads to weaker bond in these teeth. This claim is supported by some previous studies [16,39] which showed that higher concentration of fluoride in teeth resulted in more resistance to acid etching and lower surface irregularity. Both the non-fluorosed and fluorosed enamels conditioned for 120 seconds showed very irregular surfaces with definite prominences and dents (Figures 1b and 1f). the fracture pattern. [42] We expected that this enamel preparation might not be able to give adequate surface wetting. In this fracture pattern, it is easy to remove the adhesive resin from the tooth surface; however, the probability of bond failure increases at low levels of applied stress.

Conclusion
In conclusion, findings of the present study showed that enamel fluorosis significantly decreased the bond strength of orthodontic brackets. It was also found that none of the applied methods of prolonged etching, air abrasion combined with acid etching, and laser combined with acid etching could improve the SBS of metallic brackets in these teeth. Moreover, Er:YAG laser followed by acid etching significantly reduced the SBS when compared with the control acid etch group.